Combustion chambers are among other things an integral part of gas turbines, which are used in many fields for driving generators or machines. Here, the energy content of a fuel is used for generating a rotational movement of a turbine shaft. To this end, the fuel is burnt by burners in the combustion chambers connected downstream of said burners, compressed air being supplied by an air compressor.
Each burner can be assigned a separate combustion chamber, whereby it is possible for the working medium flowing out of the combustion chambers to be merged upstream of or in the turbine unit. Alternatively, the combustion chamber can also be laid out in a construction design referred to as an annular combustion chamber, in which a majority, in particular all, of the burners discharge into a common, usually ring-shaped, combustion chamber.
Burning the fuel produces a working medium under high pressure and with a high temperature. This working medium expands in the turbine unit connected downstream of the combustion chambers, performing work as it does so. To this end, the turbine unit has a number of rotatable moving blades connected to the turbine shaft. The moving blades are arranged on the turbine shaft in the form of a ring and thus form a number of rows of moving blades. The turbine also comprises a number of fixed guide vanes which are likewise fastened annularly on an inner housing of the turbine forming rows of guide vanes. The moving blades serve here to drive the turbine shaft by transferring a pulse from the working medium flowing through the turbine. Each of the guide vanes on the other hand serves to guide the flow of the working medium between two successive (viewed from the direction of flow of the working medium) moving-blade rows or moving-blade rings. A consecutive pair consisting of a ring of guide vanes or a guide-vane row and a ring of moving blades or a moving-blade row connected downstream in terms of the direction of flow of the working medium forms a turbine stage.
In the design of gas turbines of this type, one of the design goals is, in addition to the power achievable, a particularly high degree of efficiency. For thermodynamic reasons, an increase in the degree of efficiency can basically be achieved by increasing the exit temperature at which the working medium flows out of the combustion chamber and into the turbine unit. Temperatures of approximately 1200° C. to 1500° C. are therefore aimed at and also achieved for gas turbines of this type.
With such high temperatures of the working medium, however, the components and structural members exposed to this medium are exposed to high thermal loadings. However, in order to ensure with a high degree of reliability a comparatively long service life of the affected components, a design is usually required that comprises particularly heat-resistant materials and a cooling of the components concerned, in particular of the combustion chamber.
The combustion chamber wall is to this end generally furnished on its inside with an inner lining consisting of heat shield elements, which inner lining can be furnished with particularly heat-resistant protective layers and which can be cooled through the actual combustion chamber wall. To do this, a cooling procedure is generally used that is also referred to as “impact cooling”. In impact cooling, a coolant, generally cool air, is fed through a number of bore holes in the combustion chamber wall to the heat shield elements so that the coolant essentially impacts vertically onto their external surface facing the combustion chamber wall. The coolant heated up through the cooling process is then removed from the inner cavity which the combustion chamber wall forms with the heat shield elements.
In order to fasten the heat shield elements to the combustion chamber wall, there is firstly the option of connecting these to the combustion chamber wall with screws or fastening bolts. Alternatively, heat shield elements can also be anchored to the combustion chamber wall by means of appropriate holding devices onto grooves which are located in the combustion chamber wall.
A problem when operating a gas turbine is the fact that heat shield elements or even parts thereof can work loose from the combustion chamber wall. As a rule, this happens because the heat shield elements or their fastening devices are damaged by the extreme influences in the interior of the combustion chamber such as the high thermal loadings or shocks or vibrations of the combustion chamber. As a result of the flow movement of the working medium, these parts which have been loosened from the combustion chamber wall enter the turbine unit where they can destroy moving blades and guide vanes. Where there is this kind of loss of heat shield elements, loosened heat shield elements or parts thereof do not, however, enter the turbine unit since they accumulate in front of the first row of guide vanes of the first turbine stage or wedge in front of or in guide vanes of this first turbine stage. The presence of heat shield elements or parts thereof in front of the turbine unit leads, when the gas turbine is operating, to flow and pressure fluctuations in the form of flow turbulences in the turbine unit. These turbulences are generally so strong that moving blades such as in particular the moving blades of the first turbine stage snap off and thereby destroy large parts of the turbine unit, as well as the neighboring and adjoining rows of guide vane and moving blades. As a rule, in the event of a heat-shield loss, some minutes pass between the working loose of a heat shield element on the combustion chamber wall and the first breakages of moving blades, triggered by turbulences caused by jammed heat shield elements. In the event of the turbine unit being damaged, in addition to repair costs, loss-of-production costs of the gas turbine, in particular, can also accrue so that very high total costs can accrue.